Among the well-known advances in medical practice is a variety of small scale structural devices designed for surgical implantation in a patient or laboratory subject. Examples of such devices include prosthetic implants (e.g., corneal inlays or inserts, intraocular lenses, heart valves, and the like) and reparative implants (e.g., staples or rivets for attachment of delicate tissue or for closing of incisions). In general, these implants have in common with each other qualities such as biocompatibility, including chemical inertness, sterility or sterilizability; and biostability, including resistance to corrosion and degradation. Additionally, for efficient insertion and long life, any such device preferably exhibits the attributes of strength, flexibility and small size.
The present invention relates to a process for fabricating such surgical implants, and resolves many problems encountered by prior art processes. Although the present invention process is applicable to a broad genus of biocompatible, surgical implants, discussion here is limited to a species of the genus for the sake of clarity. That is, the archetype species disclosed in the following is an intraocular lens fabricated in accordance with the present invention. Naturally, one skilled in the art with knowledge of the instant disclosure can easily adapt the present invention process to numerous other implant structures known in the art.
By way of introduction, an intraocular lens has a principal refractive structure known as a lens optic, and one or more support structures for positioning and centering the lens optic within the anterior or posterior chamber of an eye. Commonly referred to as "haptics", these support structures may be integrally formed with the lens optic (a one-piece lens), or separately manufactured and attached to the lens optic (a multi-piece lens).
An important goal for intraocular lens design is to minimize trauma to the eye when the lens is inserted. To that end, effort is made to ensure, for example, that the incision to the eye is kept small during the implantation operation; that biologically inert materials are used in the construction of the intraocular lens; and that the physical proportions of the lens do not interfere, irritate, or damage delicate inner eye tissue.
What makes achieving those design goals difficult is that often the characteristics necessary for a good lens optic are undesirable for the lens haptic, and vice versa. This dichotomy presents a major challenge for designers of one-piece lenses, in which the haptics are formed integrally with the optic.
Conventional intraocular lens optics, for instance, are commonly made from biocompatible materials such as polymethylmethacrylate (PMMA). With this rather rigid material, lens optics are easily cast or machined into their final form. So in regard to handling ease and manufacturability, the benefits of PMMA are obvious. By the same token, because this material is rigid, many of the foregoing design goals are compromised.
Recently, however, more flexible materials have been devised for the lens optic. Flexible lens optics cast of elastomeric materials such as silicone or hydrogels, for example, have gained popularity because they produce foldable intraocular lenses that may be inserted through a beneficially small incision in the eye.
Once the intraocular lens is implanted, the haptics must hold the lens optic in proper alignment with the optical axis of the eye as well as support the weight of the lens optic. The haptics must therefore be sufficiently rigid to perform their function. In short, haptics must simultaneously be pliant enough to avoid damaging delicate eye tissue yet rigid enough to act as a stable support structure.
The majority of the so called "small incision" lens designs have been limited to multi-piece designs. A small incision lens connotes a flexible lens that is folded during implantation. Experience has shown that a flexible lens optic material that is desirable for the optic is usually too flimsy to work for the haptic in its support function--hence, the evolution toward the multi-piece lens design.
The type of material is also an important factor. Elastomers commonly used for the optic do not perform satisfactorily as an haptic, except, perhaps, in a broad flange configuration, which is less desirable than other more streamlined configurations. As a result, a flexible intraocular lens optic is commonly paired with more rigid polypropylene monofilament haptics.
A wide variety of haptic configurations intended for use with silicone or other elastomeric lens optics have been produced by permanent deformation of an elongated filament, as disclosed in U.S. Pat. No. 4,880,426 to Ting et al.; or by staking in the lens optic an anchor formed at an end of the filament haptic, as taught in U.S. Pat. No. 4,894,062 to Knight et al. Unfortunately, the Ting and Knight intraocular lenses exhibit only moderately satisfactory pull strengths and resistance to axial torque. As is known in the art, pull strength is a measure of the haptic's ability to resist detachment from the lens optic when subjected to an outward, radial tensile force. Torque is a twisting force applied to the haptic. Such forces, among others, are commonplace during implantation surgery where the lens may be grasped and manipulated by the haptic.
In order to obtain acceptable pull strengths, some filament haptics are provided with an enlarged anchoring head that helps secure it to a flexible lens optic. But an enlarged anchoring head is usually difficult to form consistently because conventional manufacturing techniques involve, for example, winding an end of the monofilament material around a small diameter mandrel and ultrasonically welding the overlapping part of the filament to fix the looped shape. This technique is generally disclosed in U.S. Pat. No. 4,790,846 to Christ et al. The welding is necessary because without it, the loop cannot hold its form. If the form is lost, the shape collapses or unwinds and it is easy for the haptic to detach from the lens optic. Even if the loop were welded closed, the filament might still be too flexible to retain the loop shape under tension, and again the loop would collapse.
Although the prior art looped-shape anchoring head helps interlock the haptic to the lens optic, and the design has met with some commercial success, it does have drawbacks. First, the process steps undergone in creating the looped anchoring head are extremely labor intensive, and require highly-trained technicians to skillfully manipulate intricate tools while observing through a magnifying lens. As such, it is difficult to maintain consistently high quality in the finished product. Second, because so much labor is involved, high production speeds cannot be attained. Consequently, conventional intraocular lenses of this type are not easily adapted to automated mass production, and production costs are significant.
Third, by wrapping an end of the filament around a mandrel and welding it to create the closed loop, a double thickness of haptic material at the point of overlap is made. This double thickness may be greater than the thickness of the optic itself, causing the haptic to protrude from the lens surface. In the alternative, the looped anchoring head may be positioned closer to the thicker central optical zone of the lens and away from the thinner lens periphery. Unfortunately, the presence of the anchoring head in the optical zone may distort or detract from the image seen through the lens optic.
Fourth, another disadvantage inherent in the weldedloop anchoring head haptic is the potential for the weld to break as the filament is subjected to longitudinal stress. This has been known to result in the haptic pulling away from its anchoring point and out of the optic altogether.
Fifth, insofar as the weld itself is concerned, it may be prone to chemical degredation or leaching, which may contaminate the ambient environment after implantation. Such an occurrence could be catastrophic in the eye because it may lead to vision problems.
There have been attempts at configuring other shapes for the enlarged anchoring head, aside from the weldedloop discussed above. For instance, Ting, Knight, as well as U.S. Pat. No. 4,888,013 to Ting et al. and U.S. Pat. No. 4,978,354 to Van Gent collectively disclose enlarged anchoring heads having a triangular shape, a sawtooth shape, an arrow-head shape, a knob shape, a barbed hook shape, and a hammer-head shape. The resulting haptics, however, have proven inadequate for a variety of reasons, for instance: (1) reliance on bonds that may fail or chemically leach into the environment; (2) non-adherence of optic/haptic materials; (3) an axially symmetrical anchoring head design that cannot resist torque along that rotational axis; or (4) the anchoring head shapes are too bulky.
A key to superior pull strength is the amount of surface area that the anchoring head engages in a specific direction within the lens optic. Indeed, it determines the pull strength and the ability of the haptic-optic joint to withstand torsional and bending forces.
As mentioned above, most prior art haptics rely on monofilament strands, which is a design that is saddled with many disadvantages. Departing from the filament approach, there have been attempts at acid etching the haptic from sheet material. Specifically, this process entails etching a silhouette or blank of the intended haptic design out of a sheet of biocompatible material. Unfortunately, the process cannot accurately etch out patterns having crisp 90 degree bends or sharp angles. As a result, the anchoring heads so formed have rounded edges and broadly curved structural features which easily pull from the cast soft lens optic material. Additionally, the materials used for etched haptics must be susceptible to the etching process and therefore cannot be chemically inert. The etching process limits therefore restrict a lens designer's ability to resolve structural problems by trying creative and unconventional haptic shapes or new materials.
Aside from the etching process, others have attempted to mill out an optical article from sheet stock. In particular, U.S. Pat. No. 2,302,918 to C. V. Smith discloses a method for producing optical articles. The method involves milling a blank of resinous material of predetermined circumferential dimension from sheet stock using a circular cutting device. There is, however, no disclosure or discussion about producing a structure as complex as an intraocular lens haptic, or a structure so small.